The present invention is directed to the cooling of the lubricating oil which is provided to various bearing surfaces in a screw compressor. More particularly, it relates to the cooling of oil separated from the mixture of gas and oil discharged at high temperature and pressure from a screw compressor by bringing such oil into a heat exchange relationship with compressor suction gas and ambient air.
Oil is employed for a variety of purposes in screw compressors. It is typically directed to the various compressor bearing surfaces for lubrication purposes and is injected into the working chamber of the compressor for cooling and sealing purposes.
After being used for lubrication, oil is often vented to an area of a screw compressor which is at suction pressure. Such oil is carried into and through the compressor working chamber entrained in the gas undergoing compression therein. Oil is also typically injected directly into the working chamber of a screw compressor at a location where the pressure of the medium being compressed is higher than suction pressure yet lower than discharge pressure. Such oil acts as both a sealant and coolant within the working chamber and likewise becomes entrained in the gas being compressed. Oil entrained in the gas compressed in a screw compressor must be separated from the discharge gas for continuous re-use within the compressor.
Oil used in any lubrication system will typically be more effective if cooled prior to its delivery to bearing surfaces because cooled oil is more viscous and results in higher bearing reliability. Screw compressor systems are different from typical compressor systems by virtue of the relatively very large amount of oil which is carried out of a screw compressor in the discharge gas and because such oil exits a screw compressor at relatively high temperatures and pressures.
In screw compressor applications, dedicated oil cooling apparatus and circuitry will commonly be found to exist which generally includes dedicated external heat exchangers, filters, mechanical pumps and intricate piping. Typical in this regard are the screw compressor lubrication systems disclosed in U.S. Pat. Nos. 3,708,959 and 4,497,185. Such systems are cumbersome, expensive, subject to mechanical breakdown and can cause the physical dimensions of the compressor assembly to exceed space limitations.
One common method of oil cooling in non-screw compressor applications involves bringing oil into direct heat exchange contact with compressor suction gas. Most hermetic compressors other than screw compressors are "low-side" compressors, i.e. compressors in which suction gas dumps directly into the hermetic shell of the compressor so that the interior of the shell is at low pressure. Such gas is typically in direct contact with the oil which drains to a sump area at the bottom of the compressor shell. Because sump oil in a low-side compressor is directly exposed to and is cooled by suction gas interior of the compressor's hermetic shell, it typically does not require further cooling or the employment of dedicated oil cooling apparatus. However, such convenience is unavailable in the case of screw compressors due to the discharge temperatures and pressures to which compressor lubricating oil is exposed.
U.S. Pat. No. 3,514,225 to Monden et al. discloses the immersion of a "suction cup" in the sump of a "high-side" rotary refrigeration compressor, i.e., a compressor having a housing the interior of which is at discharge pressure, for the dual purposes of cooling sump oil and for vaporizing any liquid refrigerant passing into the suction cup which might otherwise enter the compressor. The suction cup of the Monden et al. patent is a formed element interposed in a portion f the suction line found within the hermetic shell of the Monden compressor. The primary purpose of the Monden suction cup is to prevent the mechanical damage to the compressor which would result from the introduction of liquid refrigerant into the compressor's working chamber. The suction cup of Monden is a discrete element mounted internal of the hermetic shell of a compressor and is plumbed into the suction piping leading to the compressor's working chamber.
U.S. Pat. Re. No. 30,994, to Shaw, teaches the disposition of an oil carrying capillary coil in a sleeve through which suction gas is directed to the suction port of a high-side screw compressor. This small capillary line taps off of a main lubricating oil supply line and directs a small portion of the oil initially found in the lubricating oil piping to an injection point that opens into the working chamber of the compressor. The temperature of injection oil in the Shaw patent is reduced as the suction gas flows over the capillary coil disposed within the suction sleeve thereby improving the ability of the injection oil to provide a seal internal of the compressor working chamber.
The need exists to provide for the cooling of lubricating oil in a high-side screw compressor application without the use of discrete external components dedicated to the oil cooling function.